New insight into the mechanism underlying the Payne Effect - In operando X-ray photon correlation spectroscopy study

Filled rubber is widely used in automobile tires due to its mechanical reinforcement and durability, yet these materials exhibit pronounced nonlinear softening under cyclic loading, known as the Payne effect. This softening becomes more pronounced with increasing filler loading, suggesting a correlation between the Payne effect and the rupture of filler networks. However, the microscopic mechanism remains unclear due to the lack of experimental tools capable of providing the direct information at the relevant length and time scales. In this work, we employ in operando X-ray photon correlation spectroscopy (XPCS) under oscillatory strain to study the evolution of hierarchical structures and dynamics of 17 nm silica nanofillers (30 vol%, above the percolation threshold) in a crosslinked polybutadiene matrix. The measurements were performed at room temperature with strain amplitudes up to 30%, and the onset strain of the Payne effect was estimated to be 5%. During oscillatory deformation, the strain was cycled between 0% and a target value, with each plateau held for 9 s for XPCS acquisition. The XPCS data at the scattering vector, which is approximately equivalent to the distance between two aggregates connected via polymer bridges, demonstrated that the network undergoes a reversible-irreversible transition with increasing magnitude, tracking the emergence of the Payne effect. This suggests that the softening of polymer bridges is key to the Payne effect.